Cleaning Heat Exchangers: How, When, Where and How Much?

AIChE Spring Meeting 2016

ABSTRACT

Cleaning heat exchangers: how, when, where and how much?

Laura Lanchas-Fuentes, Emilio Díaz-Bejarano, Sandro Macchietto and Francesco Coletti*

Hexxcell Ltd., Imperial College Incubator, Bessemer Building Level 2, Imperial College London, SW7 2AZ (UK)

* Corresponding author: f.coletti@hexxcell.com

Energy recovery and production in oil refinery pre-heat trains are greatly affected by fouling, the progressive build-up of unwanted material on the heat exchanger surfaces. Even when good design and operating practices are followed and mitigation technologies are in place, very often crude oil fouling cannot be completely eliminated. As a result, it is necessary to carry out periodical cleaning of the heat exchangers.

Refineries typically use two methodologies to clean their heat exchangers: mechanical (e.g. hydro-blast) and chemical cleaning. Chemical cleaning methods represent a quicker and cheaper, though less effective, option than the more commonly used mechanical ones. They allow cleaning a heat exchanger in place, removing the need to open the shell and pull the bundle, thus significantly shortening the downtime. However, they typically return the exchanger to operations in a less clean condition. If the chemical used is ineffective, not only the benefits of a faster turnaround time can be lost, but the overall costs to the refinery may increase. To accurately assess the overalle economic benefits for the different options, it is therefore important to capture the trade-offs between the effectiveness of the cleaning action and the offline time required for it.

In this paper, the advantages and disadvantages between chemical and mechanical cleaning methods are discussed. Hexxcell Studio™, a thermo-hydraulic fouling analysis and prediction simulation software, is used to assess the impact of and techno-economic trade-offs between cleaning options. This cleaning analysis includes:

i)                     Establishing the effectiveness and depth of cleaning achieved by different chemical cleaning agents and the exchanger conditions left at the end (which depend on the deposit state prior to the cleaning and cleaning time). These will also affect fouling build-up in subsequent operation.

ii)                   Minimising the cleaning time for a specific chemical cleaning agent (which we term “condition-based cleaning”)

iii)                  Calculating the detailed interaction effects that cleaning individual exchangers has on the fouling, heat duty and pressure drop of other exchangers in a network and overall

iv)                  Generating alternative cleaning schedules involving the selection of the appropriate type of chemical and mechanical cleaning, and assessing the cleaning schedule performance based on thermal and hydraulic considerations, costs of energy and refinery margins.

Results are presented using an industrially relevant case study.